Long Term Evolution Systems
Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink direction and a Discrete Fourier Transform (DFT)-spread OFDM in the uplink direction. The basic LTE downlink physical resource can thus be seen as a time-frequency grid, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions may be organized into radio frames of 10 ms, with each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot, e.g., 0.5 ms, in the time domain and 12 subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction, e.g., 1.0 ms, is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a user equipment is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRBs are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain, thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information regarding which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, e.g., the control information.
From LTE Release 11 and onwards, the above described resource assignments may also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For 3GPP Release 8 to 3GPP Release 10, only Physical Downlink Control Channel (PDCCH) is available.
PDCCH
The PDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. More specifically, the DCI comprises downlink scheduling assignments, including PDSCH resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing, if applicable. A downlink scheduling assignment also includes a command for power control of the PUCCH used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.
The DCI further comprises uplink scheduling grants, including PUSCH resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH. The DCI further comprises power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.
One PDCCH carries one DCI message with one of the formats above. As multiple terminals may be scheduled simultaneously, on both downlink and uplink, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on a separate PDCCH, and consequently there are typically multiple and simultaneous PDCCH transmissions within each cell. Furthermore, to support different radio-channel conditions, link adaptation may be used, where the code rate of the PDCCH is selected to match the radio-channel conditions.
To allow for simple yet efficient processing of the control channels in the terminal, the mapping of PDCCHs to resource elements is subject to a certain structure. This structure is based on Control-Channel Elements (CCEs), which consists of nine REGs. The number of CCEs, one, two, four, or eight, required for a certain PDCCH depends on the payload size of the control information (DCI payload) and the channel-coding rate. This is used to realize link adaptation for the PDCCH; if the channel conditions for the terminal to which the PDCCH is intended are disadvantageous, a larger number of CCEs is used compared to the case of advantageous channel conditions. The number of CCEs used for a PDCCH is also referred to as the aggregation level (AL).
The network may then select different aggregation levels and PDCCH positions for different user equipments from the available PDCCH resources. For each PDCCH, a CRC is attached to each DCI message payload. The identity of the terminal (or terminals) addressed, e.g., the RNTI, is provided in the CRC calculation and not explicitly transmitted. Depending on the purpose of the DCI message, for example, unicast data transmission, power-control command, random-access response, etc., different RNTIs are used. For normal unicast data transmission, the terminal-specific C-RNTI is used.
After CRC attachment, the bits are coded with a rate-1/3 tail-biting convolutional code and rate matched to fit the amount of resources used for PDCCH transmission. After the PDCCHs to be transmitted in a given subframe have been allocated to the desired resource elements, the sequence of bits corresponding to all the PDCCH resource elements to be transmitted in the subframe, including the unused resource elements, is scrambled by a cell and subframe specific scrambling sequence to randomize inter-cell interference. Such scrambling is followed by QPSK modulation and mapping to resource elements. The entire collection of the REGs, including those unused by any PDCCH, is then interleaved across entire control region to randomize inter-cell interference as well as capturing frequency diversity for the PDCCHs.
PUCCH
If the mobile terminal has not been assigned an uplink resource for data transmission, the L1/L2 control information, e.g., channel-status reports, hybrid-ARQ acknowledgments, and scheduling requests, is transmitted in uplink resources, e.g., resource blocks, specifically assigned for uplink L1/L2 control on 3GPP Release 8 PUCCH. These resources are located at the edges of the total available cell bandwidth. Each such resource consists of 12 “subcarriers”, e.g., one resource block, within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources are frequency hopping on the slot boundary, i.e., one “resource” consists of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa. If more resources are needed for the uplink L1/L2 control signaling, e.g., in case of very large overall transmission bandwidth supporting a large number of users, additional resources blocks can be assigned next to the previously assigned resource blocks.
Carrier Aggregation
The LTE Release 10 standard has recently been standardized, supporting bandwidths larger than 20 MHz. One important requirement on LTE Release 10 is to assure backward compatibility with LTE Release 8. This should also include spectrum compatibility. That would imply that an LTE Release 10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Release 8 terminal. Each such carrier may be referred to as a Component Carrier (CC). In particular for early LTE Release 10 deployments it may be expected that there will be a smaller number of LTE Release 10 capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e., that it is possible to implement carriers where legacy terminals may be scheduled in all parts of the wideband LTE Release 10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Release 10 terminal may receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Release 8 carrier.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
During initial access a LTE Release 10 terminal behaves similar to a LTE Release 8 terminal. Upon successful connection to the network a terminal may, depending on its own capabilities and the network, be configured with additional CCs in the UL and DL. Configuration is based on RRC. Due to the heavy signaling and rather slow speed of RRC signaling, it is envisioned that a terminal may be configured with multiple CCs even though not all of them are currently used. If a terminal is configured on multiple CCs this would imply it has to monitor all DL CCs for PDCCH and PDSCH. This implies a wider receiver bandwidth, higher sampling rates, etc., resulting in high power consumption.
To mitigate the above problems, LTE Release 10 supports activation of CCs on top of configuration. The terminal monitors only configured and activated CCs for PDCCH and PDSCH. Since activation is based on Medium Access Control (MAC) control elements, which are faster than RRC signaling, activation/de-activation may follow the number of CCs that are required to fulfill the current data rate needs. Upon arrival of large data amounts multiple CCs are activated, used for data transmission, and de-activated if not needed anymore. All but one CC, the DL Primary CC (DL PCC), may be de-activated. Activation provides therefore the possibility to configure multiple CC but only activate them on a need basis. Most of the time a terminal would have one or very few CCs activated resulting in a lower reception bandwidth and thus battery consumption.
Scheduling of a CC is done on the PDCCH via downlink assignments. Control information on the PDCCH is formatted as a Downlink Control Information (DCI) message. In Release 8 a terminal only operates with one DL and one UL CC, the association between DL assignment, UL grants and the corresponding DL and UL CCs is therefore clear. In LTE Release 10 two modes of CA needs to be distinguished. The first case is very similar to the operation of multiple Release 8 terminals, a DL assignment or UL grant contained in a DCI message transmitted on a CC is either valid for the DL CC itself or for associated (either via cell-specific or UE specific linking) UL CC. A second mode of operation augments a DCI message with the Carrier Indicator Field (CIF). A DCI containing a DL assignment with CIF is valid for that DL CC indicted with CIF and a DCI containing an UL grant with CIF is valid for the indicated UL CC.
DCI messages for downlink assignments contain among others resource block assignment, modulation and coding scheme related parameters, HARQ redundancy version, etc. In addition to those parameters that relate to the actual downlink transmission most DCI formats for downlink assignments also contain a bit field for Transmit Power Control (TPC) commands. These TPC commands are used to control the uplink power control behavior of the corresponding PUCCH that is used to transmit the HARQ feedback.
In LTE Release 10, the transmission of PUCCH is mapped onto one specific uplink CC, the UL Primary CC (UL PCC). Terminals only configured with a single DL CC, which is then the DL PCC, and UL CC, which is then the UL PCC, are operating dynamic ACK/NACK on PUCCH according to 3GPP Release 8. The first Control Channel Element (CCE) used to transmit PDCCH for the DL assignment determines the dynamic ACK/NACK resource on 3GPP Release 8 PUCCH. Since only one DL CC is cell-specifically linked with the UL PCC no PUCCH collisions may occur since all PDCCH are transmitted using different first CCE.
Upon reception of DL assignments on a single Secondary CC (SCC) or reception of multiple DL assignments, a PUCCH format (which is referred to as CA PUCCH herein) that can carry the HARQ-ACK of multiple serving cells should be used. A DL SCC assignment alone is untypical. The eNB scheduler should strive to schedule a single DL CC assignment on the DL PCC and try to de-activate SCCs if not needed. A possible scenario that may occur is that eNB schedules terminal on multiple DL CCs including the PCC. If the terminal misses all but the DL PCC assignment it will use Release 8 PUCCH instead of CA PUCCH. To detect this error case eNB has to monitor both the Release 8 PUCCH and the CA PUCCH.
In LTE Release 10, the CA PUCCH format is based on the number of configured CC. Configuration of CC is based on RRC signaling. After successful reception/application of the new configuration a confirmation message is sent back making RRC signaling very safe.
CA PUCCH Transmission Scheme
In this application, CA PUCCH refers to means of transmitting HARQ-ACK of multiple serving cells in the UL. For Rel-10 LTE, CA PUCCH can be embodied in one of the following two approaches. The first method is based on the use of PUCCH format 3 that is based on DFTS-OFDM. The multiple ACK/NACK bits are encoded to form 48 coded bits. The coded bits are then scrambled with cell-specific (and possibly DFTS-OFDM symbol dependent) sequences. 24 bits are transmitted within the first slot and the other 24 bits are transmitted within the second slot. The 24 bits per slot are converted into 12 QPSK symbols, DFT precoded, spread across five DFTS-OFDM symbols and transmitted within one resource blocks (bandwidth) and five DFTS-OFDM symbols (time). The spreading sequence is user equipment specific and enables multiplexing of up to five users within the same resource blocks. For the reference signals cyclic shifted CAZAC sequences, e.g., computer optimized sequences, may be used.
The second CA PUCCH method is called channel selection. The basic principle is that the user equipment is assigned a set of PUCCH format 1a/1b resources. The user equipment then selects one of resources according to the ACK/NACK sequence the user equipment should transmit. On one of the assigned resources, the user equipment would then transmit a QPSK or BPSK. The eNB detects which resource the user equipment used and which QPSK or BPSK value the user equipment fed back on the used resource and combines this into a HARQ response for associated DL cells. A similar type of mapping including a bundling approach is also done for TDD as in the FDD, in case the user equipment is configured with channel selection.
Time Division Duplex
Transmission and reception from a node, e.g., a terminal or user equipment 501 and base station 401 in a cellular system such as LTE, may be multiplexed in the frequency domain or in the time domain (or combinations thereof). Frequency Division Duplex (FDD) as illustrated to the left in FIG. 1 implies that downlink and uplink transmission take place in different, sufficiently separated, frequency bands. Time Division Duplex (TDD), as illustrated to the right in FIG. 1, implies that downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum.
Typically, the structure of the transmitted signal in a communication system is organized in the form of a frame structure. For example, LTE uses ten equally-sized subframes of length 1 ms per radio frame as illustrated in FIG. 2.
In case of FDD operation, illustrated in the upper section of FIG. 2, there are two carrier frequencies, one for uplink transmission (fUL) and one for downlink transmission (fDL). At least with respect to the terminal in a cellular communication system, FDD may be either full duplex or half duplex. In the full duplex case, a terminal may transmit and receive simultaneously, while in half-duplex operation, the terminal may not transmit and receive simultaneously. The base station is capable of simultaneous reception/transmission though, e.g., receiving from one terminal while simultaneously transmitting to another terminal. In LTE, a half-duplex terminal is monitoring/receiving in the downlink except when explicitly being instructed to transmit in a certain subframe.
In case of TDD operation, illustrated in the lower section of FIG. 2, there is only a single carrier frequency and uplink and downlink transmissions are always separated in time also on a cell basis. As the same carrier frequency is used for uplink and downlink transmission, both the base station and the mobile terminals need to switch from transmission to reception and vice versa. An essential aspect of any TDD system is to provide the possibility for a sufficiently large guard time where neither downlink nor uplink transmissions occur. This is required to avoid interference between uplink and downlink transmissions. For LTE, this guard time is provided by special subframes, e.g., subframe 1 and, in some cases, subframe 6, which are split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). The remaining subframes are either allocated to uplink or downlink transmission.
TDD allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. In LTE, there are seven different configurations as shown in FIG. 3. It should be appreciated that a DL subframe may mean either DL or the special subframe.
To avoid severe interference between downlink and uplink transmissions between different cells, neighbor cells should have the same downlink/uplink configuration. If this is not done, uplink transmission in one cell may interfere with downlink transmission in the neighboring cell and vice versa. Hence, the downlink/uplink asymmetry may typically not vary between cells, but is signaled as part of the system information and remains fixed for a long period of time.
TDD HARQ Control Timing
The timings for HARQ A/N feedbacks for the PDSCH are specified with extensive tables and procedure descriptions for each U/D configuration in Table 1. The user equipment shall also feedback PDSCH decoding A/N information in pre-defined UL subframes. The user equipment shall transmit such HARQ A/N responses on the PUCCH in UL subframe n if there is PDSCH transmission indicated by the detection of corresponding PDCCH or there is PDCCH indicating downlink SPS release within subframe(s) n−k, where k is within the association set K={k0, k1, . . . , kM−1} listed in Table 1.
TABLE 1Downlink association set index K = {k0, k1, . . . , kM−1} for TDDUL-DLSubframe nConfiguration01234567890646417, 647, 6428, 7, 4, 68, 7, 4, 637, 6, 116, 55, 4412, 8, 7, 116, 5, 4, 7513, 12, 9, 8, 7, 5, 4,11, 6677577
Examples to illustrate the timing are in reference to FIG. 4A. It should be appreciated that the leftmost subframe is denoted as subframe 0 and the rightmost subframe is denoted as subframe 9. The subframe numbers have been provided in FIG. 4A for the purpose of explanation. For the UL subframe 7 in the configuration 1 cell, Table 1 shows K={7,6}, which corresponds to carrying possible HARQ A/N feedbacks for PDSCHs transmitted in subframes 7−7=0 and 7−6=1 (n−k). This is illustrated as arrows originating from DL subframes 0 and 1, being directed towards the UL subframe 7 in FIG. 4A, Configuration #1.
Similarly, for the UL subframe 2 in the configuration 2 cell, as illustrated in FIG. 4B, Table 1 shows K={8, 7, 4, 6}, which corresponding to carrying possible HARQ A/N feedbacks for PDSCHs transmitted in subframes 4, 5, 6, and 8 of the preceding frame. This is illustrated as arrows originating from these DL subframes are directed towards the UL subframe 2 in FIG. 4B, Configuration #2. It should be appreciated that in the examples provided herein, the n−k calculation is a modular 10 calculation.